1. Field of the Invention
This invention generally relates to light harvesting porphyrin polymer materials and, more particularly, to a metal (M)-meso-tetraphenylporphyrin polymer (M-poly-meso-TPP) and associated synthesis method.
2. Description of the Related Art
Although chlorophyll, chlorophyll derivatives, and synthetic porphyrins have diverse molecular structures, they exhibit characteristic optical properties over comparable wavelength ranges (typically λ=350 to 700 nm in many cases). Synthetic porphyrins (and corresponding metalloporphyrins) consist of a conjugated 22π electron system, 18 of which are effectively delocalized to fit the Hückel requirement for aromaticity. In addition to their structural resemblance to natural chromophores such as chlorophyll, synthetic porphyrins are attractive candidates as light-harvesting materials due to their high structural stability, robust light absorption capabilities, and synthetic accessibility, as compared to more complex, naturally occurring chromophores. Photo-excited processes involving porphyrins are facilitated by the highly delocalized n-system, which is capable of resisting major structural changes upon oxidation. Most importantly, the redox properties of porphyrins and metalloporphyrins are dramatically altered upon photo-excitation, which leads to the generation of porphyrin excited states that can be exploited in photovoltaic (PV) applications.
The ability of porphyrins to efficiently harvest light over broad wavelength ranges has generated significant interest in their potential in solar energy applications over the last few decades. As a result, synthetic protocols towards the fabrication of “customized” porphyrin architectures have become well-established and have been widely adopted as conventional methods. In general, the electronic properties of porphyrins can be altered using a number of strategies including some of the following: functionalization and/or modification along the porphyrin periphery, insertion of transition metals into the macrocyclic core, and coordination of metalloporphyrins with various ligands.
Although many alternatives have evolved, there exist two major fundamental synthetic strategies for constructing the core porphyrin macrocycle, which are described independently below.
In the first scenario (Method A), an aromatic aldehyde (for example, benzaldehyde in the case of tetraphenylporphyrin) or mixture of chemically functionalized aromatic aldehydes is reacted with pyrrole (or chemically functionalized pyrroles) using an acid catalyzed condensation reaction performed in organic acid (acetic acid or propionic acid, for example) at elevated temperatures. Since the distribution of chemical reaction products (porphyrins) consists of a statistical mixture representing all possible combinations at different ratios, in addition to polymeric and oligomeric products, the usefulness is ordinarily limited to symmetrical tetraphenylporphyrins. In addition, the sensitivity of the starting materials to the rather harsh reaction conditions (high temperature in acidic media) necessarily places limitations on the nature of reactants that can be utilized. Assuming the target porphyrin is an unsymmetrical tetraphenylporphyrin, subsequent purification is almost always a requirement. In almost every case, the necessary purification involves chromatographic separation of a complex mixture of porphyrin materials. Although overall reaction yields using this strategy are typically only modest, starting materials are often commercially available.
As used herein, chromatography refers to a purification method that involves separating/isolating individual compounds from a mixture of compounds. The most common chromatography technique is column chromatography whereby a vertical (usually glass) column is packed with a stationary phase (usually silica gel or alumina) that functions as an adsorbent material. Typically, a material or mixture of materials (dissolved in a solvent) is placed onto the top of the stationary phase in the column and is allowed to proceed down the column by addition of a mobile phase (commonly referred to as eluent). The column chromatography process may be performed by relying on gravity (or percolation) to flow the mobile phase down the column or by applying a positive pressure in a technique commonly referred to as “flash” chromatography. During the chromatography process, an equilibrium is established between the solute (which refers to the materials/compounds to be separated) adsorbed on the stationary phase and the mobile phase flowing down the column. Owing to the differences in interactions with the stationary and mobile phases, individual components of a mixture will move down the column at different rates (partitioning), thereby allowing separation of a mixture into individual components. Typically, the eluent is collected at the bottom of the column in fractions corresponding to individual materials obtained from separation of the original mixture. Overall, the effectiveness of a chromatographic separation to provide a single component in pure form is dependent upon a number of factors including choice of stationary and mobile phases (solvent polarity, for example), partitioning coefficients for components in the original mixture, quantity of mixture to be subjected to the method at one time, and dimensions of the column in which the separation is performed, among others.
In the second strategy (Method B), the porphyrin macrocycle is constructed under milder reaction conditions which is facilitated by catalytic amounts of strong acid (trifluoroacetic acid (TFA), for example) or Lewis acid (boron trifluoride (BF3), for example) in organic solvents at ambient conditions or only slightly elevated temperatures, and is usually performed under inert atmosphere (nitrogen). In many cases, an appropriate oxidizing agent (p-chloranil or similar) is added following condensation to facilitate porphyrin formation from the intermediate porphyrinogen species. In general, the milder reaction conditions are better tolerated by a wider range of starting materials, although certain functional groups still require appropriate (chemical) protection. Conveniently, elaborate porphyrin materials are more accessible due to the inherent synthetic flexibility of this approach. Although aldehydes and pyrroles may still function as the fundamental synthetic building blocks, dipyrrylmethanes, which represent a reaction product of one aldehyde with two pyrrole units, can be prepared and subsequently employed as a starting material for porphyrin formation. In this way, the amount of desired porphyrin product formed can be maximized (within statistical limits) while side-reactions are suppressed, at least to an extent. In addition to the synthesis of tetraphenylporphyrins, this strategy can be extended to the preparation of mono-, di-, and triphenylporphyrins, among others. In spite of these advantages, subsequent purification of the reaction mixture still requires chromatographic methods. In addition, the preparation of the dipyrrlmethane starting materials can prove difficult and requires analogous purification methods. Nevertheless, reaction yields for porphyrins synthesized through these methods can approach 50% in some cases. Subsequent deprotection of functional groups (if used as starting material for porphyrin formation) necessitates an additional synthetic chemistry step as well as further purification, often via column chromatography.
The chemistry outlined in the two preceding methods leads to the formation of an intact porphyrin macrocycle. More often than not (at this stage), the porphyrin product functions as a scaffolding upon which additional chemical functionalities are introduced through an array of available synthetic methodologies. Such modifications, which may include the introduction of additional functional groups along the porphyrin periphery and/or insertion of transition metals in the porphyrin core, necessarily require additional synthetic steps. Each subsequent modification of the porphyrin macrocycle requires chemical reaction(s) followed by a purification step, which in most cases involves column chromatography and/or recrystallization. In addition, many of these chemical reactions require highly controlled conditions (dry solvents, inert atmosphere, etc.) and/or extended reaction times (up to 48 hours or more) and are further complicated by the fact that conversion percentages (from porphyrin starting material(s) to desired porphyrin product) can vary from extremely low to moderate, while almost never furnishing a single porphyrin product. As a result of this, chromatographic separation of a mixture of porphyrin products or, at the very least, separation of desired porphyrin product from unreacted porphyrin starting material, is required. For these reasons, only modest amounts of final porphyrin material are most often obtained following a sequence of multiple synthetic steps.
As previously mentioned, the motivation to synthesize more elaborate porphyrin architectures is correlated with a desired function or performance. In simple cases, straightforward modifications are performed for the purposes of providing “attachment” to other molecules, substrates and/or other porphyrins including the introduction of carboxyl, sulfonate, or phosphonate groups for adsorption onto metal oxides (dye-sensitized solar cells, DSCs) or introduction of an aldehyde group, as is the case for constructing molecular electronic devices involving porphyrins and fullerenes or carbon nanotubes through the well-known Prato reaction.
Often, the desired enhancements accessible through chemical modification of porphyrins involve manipulation of light-harvesting properties and/or excited-state behaviors (electron transfer). For example, it is well-known that increasing the n-conjugation extending from the porphyrin core can lead to enhanced absorption properties which may include (1) increased absorptivity over a particular wavelength range, (2) a broadening of optical absorption over wider ranges and/or (3) shifting of absorption towards longer (or shorter) wavelengths. Not surprisingly, this can be accomplished through a number of synthetic approaches and may involve direct attachment of conjugated moieties to the porphyrin core, formation of larger, porphyrin-centered macrocyclic derivatives prepared through ring-fusion reactions, or linear (or branched) poly-porphyrin architectures through which networks (dimers, trimers, tetramers, oligomers, polymers) of porphyrin subunits are connected through various motifs. Most often, porphyrin dimers, trimers, and tetramers are constructed in a step-wise approach whereby single porphyrin units are sequentially added to the evolving tetramer. In these cases, the construction of the final porphyrin arrays involves: (1) synthesis of monomeric porphyrin subunits (or multiple subunits), (2) chemical reaction(s) to “link” two individual subunits, (3) purification of the new molecule and, finally, (4) a repeat of steps (1)-(3). Although precise control over subunit reactivity and overall configuration is possible, this approach is both challenging and time-consuming while often leading to only modest quantities of final porphyrin material. Nevertheless, elaborate porphyrin architectures can indeed been realized using these methodologies.
FIGS. 1A through 1C respectively depict the molecular structures of a β-zinc porphyrin dimer, meso-zinc porphyrin dimer, and meso-zinc porphyrin trimer (prior art). Lin et al. described the synthesis and photophysical studies of conjugated acetylenic porphyrin arrays prepared through metal-mediated and/or organometallic coupling reactions of porphyrin monomers.1 To summarize briefly, zinc porphyrins containing acetylene units attached to either the β- or meso-positions were synthesized from the appropriate zinc porphyrin starting materials under controlled conditions. The β- and meso-substituted zinc porphyrin monomers were used for the construction of a β-zinc porphyrin dimer, meso-zinc porphyrin dimer, and meso-zinc porphyrin trimer. Overall, the optical absorption properties of the dimeric (and trimeric) porphyrin arrays were dramatically altered with respect to the corresponding monomeric porphyrins.
FIGS. 2A and 2B depict, respectively, the molecular structures of a phenylbutadiyne-linked zinc porphyrin dimer and phenylacetylene-linked zinc porphyrin trimer (prior art). Wagner et al. described the synthesis of phenylacetylene and phenylbutadiyne-linked porphyrin dimers and trimers using mild, copper-free conditions.2 The dimers (or trimers) were formed in variable yields using either (1) a combination of two different synthetic porphyrins (for dimer) or (2) the reaction of one bi-functional porphyrin unit with two of the same porphyrins (for trimer). Not surprisingly, the distribution and overall yields of reaction products were highly variable and the complex mixtures of reaction products required subsequent purification and structural elucidation.
As an alternative to dimers, trimers, etc., large polymeric porphyrin architectures (linear or branched) are accessible using conventional synthetic methodologies with the appropriate monomeric porphyrins. Although polymeric porphyrins exhibit enhanced optical and electronic properties, overall solubility tends to be a limiting factor for practical application. In addition, the fabrication of porphyrin polymers often entails specialized equipment (electropolymerization) and/or glove-boxes/Schlenk lines (organometallic chemistry), highly-controlled chemical reaction conditions (chemical reaction stoichiometry, temperature) and, in many cases, extended periods of time. Nevertheless, the formation of extended networks (or polymers) of porphyrin-based “absorbers” is a promising strategy for fabricating both strongly and broadly absorbing materials.
Therein et al. described the preparation of highly-conjugated porphyrin arrays using metal-mediated cross-coupling reactions with metalloporphyrins.3 The approach(es) involve porphyrin polymer formation using classical organometallic and/or oxidative coupling chemistry. The formation of both linear and branched poly-porphyrin architectures is indeed feasible using the approaches described although major drawbacks of the technology include: (1) the need to carefully synthesize and purify the appropriate porphyrin monomers, (2) critical control of reaction parameters (Schlenk line) in order for the necessary chemical reactions to be carried out in some cases, and (3) the requirement of heating under inert atmospheric conditions for extended periods of time.
In addition to synthetic methodologies described above, Chen et al. reported the synthesis of porphyrin-containing polymers linked through triazole rings prepared either by click polymerization with Cu(I) catalyst or heating to afford materials exhibiting relatively high molecular weight.4 Overall, polymers obtained using the Cu(I)-catalyzed pathway provided lower molecular weights due to reduced rates for molecular weight growth compared to the metal free (thermal) polymerization. Overall, the polymers exhibited solubility in common organic solvents and demonstrated thermal stability up to 350° C. Conveniently, the method offers advantages in terms of reduced by-product formation as a result of the polymerization process. Jyothish et al. provided the preparation of a series of tris(arylmethyl)ammonium-coordinated molybdenum(VI) propylidyne catalysts that enabled the efficient synthesis of ethynylene-bridged porphyrin-based arylene ethynylene polymers through alkyne metathesis.5 Xiang et al. described the synthesis of two conjugated polymers consisting of alternating main chain structures of zinc porphyrin-terthiophene (P-PTT) and zinc porphyrin-oligothiophene (P-POT).6 The introduction of thiophene units in the meso-aryl positions of the porphyrin provided a bathochromic shift and broadened optical absorption characteristics relative to the monomeric zinc porphyrin (PZn) in both solution and thin solid film. Furthermore, electrochemical investigation indicated appropriate energy levels for efficient charge transfer and separation at the polymer (donor) and PCBM (acceptor) interface. Finally, bulk heterojunction (BHJ) solar cells based on P-PTT and PPOT demonstrated power conversion efficiencies (PCEs) up to 0.32% and 0.18%, respectively.
Shi et al. provided a donor-acceptor porphyrin-containing conjugated co-polymer (PCTTQP) that exhibited broad absorption along the visible spectrum.7 Furthermore, corresponding BHJ solar cells based upon blends of PCTTQP and PC71BM demonstrated PCE=2.5%. Natori et al. reported the synthesis of a tetraphenylporphyrin (H2TPP)-end-functionalized poly(p-phenylene) (H2TPP-PPP) exhibiting a well-controlled polymer chain structure and broad absorption across the visible region.8 Finally, Wirotius et al. described the fabrication of dendrimer-like star-branched poly(ethylene oxide)s (PEOs) comprising two and three generations with Zn(II) tetraphenylporphyrin (ZnTPP) moieties located at both the core and at each branching point through a convergent approach using “click chemistry”.9 
An alternative to chemical methods for the fabrication of porphyrin polymers involves direct electrochemical polymerization of the appropriate porphyrin subunits (monomers).10-11 Giraudeau et al. demonstrated a convenient method for the electropolymerization of porphyrins that circumvents the difficulties involved in synthesizing a functional porphyrin monomer by directly employing a commercial zinc-β-octaethylporphyrin (ZnOEP) with 4,4′-bipyridine (bpy) in solution.12 
FIG. 3 depicts a structure of a porphyrin polymer prepared via electropolymerization of porphyrin monomers (prior art). Gust et al. reported the in situ electropolymerization of porphyrins and porphyrin-[60]fullerene dyads to form conjugated porphyrin-based materials on conductive electrodes.13-14 With respect to the porphyrin moieties in the polymers, hole conductivity proceeds in ways analogous to conduction in polyaniline. In all cases, the porphyrin units are linked by aminophenyl groups to form a linear chain in which the porphyrins function as an integral part of the polymer backbone. In general, the absorption characteristics of the films resemble that of the corresponding porphyrin subunits, except for significant broadening of absorption peaks due to extended conjugation in the polymers. In this case, the insolubility of the polymerized porphyrin polymer is circumvented by an in situ electropolymerization process that deposits the polymer directly onto an electrode. Nevertheless, there exist some limitations with this technology which include: (1) the need to strategically synthesize the appropriate porphyrin monomer through several independent steps, and (2) the electropolymerization process proceeds only within a controlled environment.
FIG. 4A through 4C respectively depict porphyrin dimers consisting of two free-base porphyrins, two zinc porphyrins, and one free-base porphyrin and one zinc porphyrin, whereby Im=imidazole (prior art). In contrast to chemical methods that furnish extended, covalently-bonded porphyrin networks, porphyrin polymers have been realized through effective “self-assembly” in solution. Michelsen et al. demonstrated the ability to control the self-assembly of polymers formed from cobalt porphyrins functionalized with two pyridine ligands.15 Accordingly, high molecular masses (>100 kDa) could be realized for the polymer which could be modulated by adding monofunctional chain terminating moieties or by adjusting the concentration. Kobuke et al. described the design and synthesis of alkyne-linked, bis(imidazoyl)porphyrin complex (dimer) arrays that exhibited large, dual photon absorption characteristics.16 The core technology consists of covalently linking two porphyrins bearing imidazole groups together (to form dimers) through an alkyne linker. The three possible combinations are as follows: (1) both porphyrins may be free-base (no metal), (2) both porphyrins may be metallated (zinc), or (3) one porphyrin may be free-base while the other porphyrin is metallated as indicated in FIGS. 4A-4C. As is common with zinc porphyrins, axial coordination readily occurs in the presence of amine groups possessing a free electron pair. Not surprisingly, enhanced redox behaviors are commonly observed upon axial coordination of various ligands to metalloporphyrins, although the phenomenon is sensitive to a number of factors that include the mode/strength of coordination, solvent properties, and concentration in addition to geometric considerations.
Although porphyrin polymers have significant relevance as absorber materials in PV applications, several reports describe the gas adsorption properties and catalytic activity of functional porphyrin networks, which are briefly summarized below. Wang et al. described the synthesis of four porous polymers consisting of nickel (Ni)-porphyrin units through which Brunauer-Emmet-Teller (BET) specific surface areas up to 1711 m2/g were achieved.17 Modak et al. provided a synthesis of iron (Fe)-containing, porous organic polymers (POPs) through a one-pot, bottom-up approach.18 Overall, the Fe-POPs exhibited both high BET surface area with large micropores and demonstrated excellent CO2 capture. Finally, Shultz et al. demonstrated the synthesis of a POP containing free-base porphyrin subunits through condensation of a bis(phthalic acid)porphyrin with tetra(4-aminophenyl)methane.19 Subsequent metallation provided microporous materials incorporating either Fe or manganese (Mn) porphyrins that demonstrated catalytic activity in both olefin epoxidation and alkane hydroxylation.
In spite of the novel technologies described herein, it would be advantageous to develop methodologies for (poly)porphyrin-based “absorber” materials which meet the following criteria:                (1) Consist of porphyrin subunits (monomers) that can be strategically synthesized in a straightforward manner:                    a. Synthesis should be performed in a limited number of steps to furnish a functional, polymerizable porphyrin monomer without the use of expensive catalysts (palladium, for example) at any step in the synthesis.            b. Purification following each synthetic step should be simplified (filtration as opposed to column chromatography).            c. Reaction yields (conversion of starting material(s) to product) should be maximized, preferably quantitative, for some (if not all) of the chemical reactions involved.            d. Synthesis should be performed without the need for controlled environments (moisture-free, oxygen-free, etc.) and should not require any special equipment.            e. Chemical reaction times for the synthesis of porphyrin monomers and polymers should be reasonably short overall and not require extended periods of heating.                        (2) Porphyrin polymer should demonstrate enhanced optical properties (absorption, for example) relative to the monomer subunit(s).        (3) Porphyrin polymer should be amenable to appropriate processing. Due to insolubility concerns, the strategies and methods for enhancing the solubility should be compatible with the core process for fabricating the polymer.            1. V. S-Y. Lin, S. G. DiMagno and M. J. Therien, “Highly Conjugated, Acetylenyl Bridged Porphyrins: New Models for Light-Harvesting Antenna Systems”, Science 1994, 264, 1105-1111.    2. R. W. Wagner, T. E. Johnson, F. Li and J. S. Lindsey, “Synthesis of Ethyne-Linked or Butadiyne-Linked Porphyrin Arrays Using Mild, Copper-Free, Pd-Mediated Coupling Reactions”, Journal of Organic Chemistry 1995, 60, 5266-5273.    3. M. J. Therien and S. G. DiMagno, “Metal-Mediated Cross-Coupling with Ring-Metalated Porphyrins”, U.S. Pat. No. 5,986,090, 1999.    4. H. Chen, J. Zeng, F. Deng, X. Luo, Z. Lei and H. Li, “Synthesis and Photophysical Properties of Porphyrin-Containing Polymers”, Journal of Polymer Research 2012, 19:9880.    5. K. Jyothish, Q. Wang and W. Zhang, “Highly Active Multidentate Alkyne Metathesis Catalysts: Ligand-Activity Relationship and Their Applications in Efficient Synthesis of Porphyrin-Based Aryleneethynylene Polymers”, Advanced Synthesis & Catalysis 2012, 354, 2073-2078.    6. N. Xiang, Y. Liu, W. Zhou, H. Huang, X. Guo, Z. Tan, B. Zhao, P. Shen and S. Tan, “Synthesis and Characterization of Porphyrin-Terthiophene and Oligothiophene n-Conjugated Copolymers for Polymer Solar Cells”, European Polymer Journal 2010, 46, 1084-1092.    7. S. Shi, X. Wang, Y. Sun, S. Chen, X. Li, Y. Li and H. Wang “Porphyrin-Containing D-π-A Conjugated Polymer with Absorption Over the Entire Spectrum of Visible Light and Its Applications in Solar Cells”, Journal of Materials Chemistry 2012, 22, 11006-11008.    8. I. Natori, S. Natori, A. Kanasashi, K. Tsuchiya and K. Ogino, “Synthesis of Porphyrin-End-Functionalized Poly(p-Phenylene) as a Leaf-Green-Colored Soluble Semiconducting Polymer: Control of π-π Interactions for Visible-Light Absorption”, Reactive and Functional Polymers 2013, 73, 200-206.    9. A-L. Wirotius, E. Ibarboure, L. Scarpantonio, M. Schappacher, N. D. McClenaghan and A. Deffieux, “Hydrosoluble Dendritic Poly(ethylene oxide)s with Zinc Tetraphenylporphyrin Branching Points as Photosensitizers”, Polymer Chemistry 2013, 4, 1903-1912.    10. L. Ruhlmann, A. Schulz, A. Giraudeau, C. Messerschmidt and J-H. Fuhrhop, “A Polycationic Zinc-5,15-Dichlorooctaethylporphyrinate-Viologen Wire”, Journal of the American Chemical Society 1999, 121, 6664-6667.    11. J. Hao, A. Giraudeau, Z. Ping and L. Ruhlmann, “Supramolecular Assemblies Obtained by Large Counteranion Incorporation in a Well-Oriented Polycationic Copolymer”, Langmuir 2008, 24, 1600-1603.    12. A. Giraudeau, D. Schaming, J. Hao, R. Farha, M. Goldmann and L. Ruhlmann, “A Simple Way for the Electropolymerization of Porphyrins”, Journal of Electroanalytical Chemistry 2010, 638, 70-75.    13. P. A. Liddell, M. Gervaldo, J. W. Bridgewater, A. E. Keirstead, S. Lin; T. A. Moore, A. L. Moore and D. Gust, “Porphyrin-Based Hole Conducting Polymer”, Chemistry of Materials 2008, 20, 135-142.    14. J. D. Gust, P. A. Liddell, M. A. Gervaldo, J. W. Bridgewater, B. J. Brennan; T. A. Moore and A. L. Moore, “Electrically Conducting Porphyrin and Porphyrin-Fullerene Electropolymers”, US2010/0065123 A1, 2010.    15. U. Michelsen and C. A. Hunter, “Self-Assembled Porphyrin Polymers”, Angewandte Chemie International Edition 2000, 39, 764-767.    16. Y. Kobuke and K. Ogawa, “Porphyrin Array Exhibiting Large Two Photon Absorption Property and Including, as Structural Unit, Bis(Imidazolylporphyrin Metal Complex) Linked with Acetylenic Bond and the Derivative Thereof, and Method of Producing the Same”, U.S. Pat. No. 7,022,840 B2, 2006.    17. Z. Wang, S. Yuan, A. Mason, B. Reprogle, D-J. Liu and L. Yu, “Nanoporous Porphyrin Polymers for Gas Storage and Separation”, Macromolecules 2012, 45, 7413-7419.    18. A. Modak, M. Nandi, J. Mondal and A. Bhaumik, “Porphyrin Based Porous Organic Polymers: Novel Synthetic Strategy and Exceptionally High CO2 Adsorption Capacity”, Chemical Communications 2012, 48, 248-250.    19. A. M. Shultz, O. K. Farha, J. T. Hupp and S. T. Nguyen, “Synthesis of Catalytically Active Porous Organic Polymers from Metalloporphyrin Building Blocks”, Chemical Science 2011, 2, 686-689.